This invention is concerned generally with gas chromatography, and more particularly with the oven that contains the separating colomns in a gas chromatograph system.
A gas chromatograph oven usually comprises a thermally insulated housing internally accessable through a door, a heating element, and a motor driven fan for stirring the air in the housing. The stirring fan continuously mixes the air within the oven to minimize temperature gradients which could adversely affect the performance of the chemical process within the separating columns.
An important requirement of a gas chromatograph oven is that it be capable of rapid cooling from a higher temperature to a closesly maintained lower temperature. Typically this has been accomplished by either opening the oven door to permit the interior air to be cooled by the ambient room temperature air, or by cryoinjection of a coolant such as liquid CO.sub.2 or N.sub.2 into the oven. These methods, as presently utilized, have several disadvantages.
To utilize the ambient room temperature air for cooling, the door must be set ajar either manually or automatically. A preprogrammed control mechanism is used to open and close the door as necessary in the automatic mode. Sometimes, an auxilary blower is used to aid in mixing the cooler ambient room temperature air with the warmer air within the oven. When the oven door is opened partially, a turbulent air region is created around this opening by the interaction of the hot air which tries to escape from the oven and the room temperature air which tries to replace the hot air. The result is undesirably large temperature variations with time and from point to point in the chromatographic separation column area of the oven. These temperature variations are detrimental to the reproducibility of chromatographic separations, hence chromatographic separations should not be carried out with the door ajar. Necessitating that the oven door be closed results in two disadvantages. First, the closed oven temperature will stabilize typically no lower than 25.degree. C above ambient room temperature without power applied to the oven heater. The elevated oven temperature is generated by friction between the rotating stirring fan and the circulating air within the oven. Additional oven heat is supplied by auxiliary heated devices, e.g. the independently heated cryoinjection mechanism. At temperatures between ambient and 25.degree. C above ambient chromatographic separations cannot typically be carried out without the use of cryogenic coolants. Second, the heat retained by the walls of the oven necessitate the cooling of the oven air far below the new control temperature range before the door is closed. The additional cooling is necessary since the heat stored in the walls of the oven can raise the oven temperature considerably. Thus, the time between chromatographic separations is much longer than would be required to cool the oven air from the higher temperature to the lower temperature.
Sometimes the rate of cooling of the oven is improved by the use of an auxiliary blower, but the use of such an additional component is undesirable because of the additional cost and maintenance potential. The open, or opening oven door also presents a safety hazard to the operator since under preprogrammed control the door may open unexpectedly, and when open it may present an obstacle to the free movement of the operator, as well as exposing him to potentially hot interior oven surfaces on the order of 200.degree. C, for example.
The use of coolants injected into the oven for cooling the internal air to near ambient temperatures (between ambient and about 25.degree. C above ambient) is undesirable, although required for operation at sub-ambient temperatures. From the standpoint of cost and safety, coolant injection presents the need for a valve, feed lines, and liquid CO.sub.2 or N.sub.2 holding tanks which are expensive and present obstacles to the operator. Also, temperature transients and gradients between different locations within the oven are relatively large when liquid coolant is used, resulting in a loss of chromatographic separation consistency.